The throughput of wireless communication systems has increased significantly by new technologies introduced in LTE and Wi-Fi. These technologies, however, are not sufficient to meet the demands of future applications which will require Gbits/sec of throughput and latencies of 1 ms. Therefore, research on a new radio access technology, known as the 5G, has already started. As the applications and ubiquity of cellular communication systems grow, they are expected to support new features, and meet a more stringent set of performance requirements. Based on the general requirements set out by ITU-R (as discussed in ITU-R Recommendation M.2083, “IMT vision—framework and overall objectives of the future development of IMT for 2020 and beyond,” 2015), NGMN (as discussed in NGMN Alliance, “5G white paper,” 2015), and 3GPP, a broad classification of the use cases for emerging 5G systems can be depicted as follows: Enhanced Mobile Broadband (eMBB), Massive Machine Type Communications (mMTC) and Ultra Reliable and Low latency Communications (URLLC). Different use cases may focus on different requirements such as higher data rate, higher spectral efficiency, low power and higher energy efficiency, lower latency and higher reliability.
Massive machine type communication (mMTC) is one of the three main use case categories for the fifth-generation cellular standards (5G). The mMTC use case is characterized by the desire to provide connectivity for a very large number of low cost devices. Target applications include things such as smart metering, home appliances, and remote sensors. Common to all of these applications is that the data transmissions are relatively small and infrequent. One of the requirements to make these massive deployments feasible will be the ability to use low cost devices with significantly extended battery life.
As compared to conventional multiple-input and multiple-output (MIMO), spatial modulation MIMO (SM-MIMO) is a modulation technique that modulates information onto the antenna indices at the transmitter allowing the number of radio frequency (RF) chains to be less than the number of transmit antennas, thus reducing overall cost and power consumption. Therefore, SM-MIMO primarily targets energy efficiency (EE) over spectral efficiency (SE).
Link adaptation is a widely used technique whereby certain transmit parameters are dynamically configured, based on channel conditions, in order to optimize certain link criteria. Adaptive modulation and coding (AMC) is one common link adaptation scheme that adjusts the modulation and coding scheme based on the current channel conditions and a desired error probability so that the spectral efficiency (SE) is maximized. Multiple input multiple output (MIMO) technology also primarily targets higher SE. Spatial multiplexing (SMX) is a MIMO technique which allows for multiple simultaneous data streams to be transmitted and received over the same radio channel. For this technique to be successful certain channel conditions should be satisfied, hence link adaptation can also be applied by dynamically adjusting the SMX mode based on the current channel conditions so as to maximize the SE.
To summarize, SM-MIMO is a powerful communication technique that primarily targets low cost devices and energy efficient operation. Furthermore, link adaptation is a similarly powerful technique that is used to increase SE based on the changing channel conditions these systems will inevitably encounter.
Electrically reconfigurable antennas are capable of dynamically reshaping themselves and thereby changing their radiation characteristics. This dynamic reshaping can be realized by integrating PIN/varactor diodes and/or MEMS devices into the structure of the antenna, and further electrically controlling these components. Sample reconfigurable antennas are shown in FIG. 2. Reconfigurable antennas can be classified into four different categories (as discussed in C. G. Christodoulou, Y. Tawk, S. A. Lane, and S. R. Erwin, “Reconfigurable antennas for wireless and space applications,” Proceedings of the IEEE, vol. 100, no. 7, pp. 2250-2261, 2012). A frequency reconfigurable antenna is a radiating structure that is able to change its operating or notch frequency by hopping between different frequency bands. This is achieved by producing some tuning or notch in the antenna reflection coefficient. A radiation pattern reconfigurable antenna is able to tune its radiation pattern in terms of shape, direction, or gain. A polarization reconfigurable antenna is a radiating structure that can change its polarization (horizontal/vertical, _slant 45_, left-hand or right-hand circular polarized, etc.) Such an antenna can change, for example, from vertical to left-hand circular polarization. A further category is a combination of the previous three categories. For example, one can achieve a frequency reconfigurable antenna with polarization diversity at the same time.